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Cosmochemistry NAG5-7285 Final Report Thomas G. Sharp
NASA Cosmochemistry Grant NAG5-7285 Final Report
Mineralogy and microstructures of chock-induced melt veins in chondrites
Thomas G. Sharp
Grant period 5/1/98 - 4/30/00
1. Introduction
Shock metamorphism results from high-velocity collisions on planetary bodies and
therefore represents a fundamental process in the formation of meteorites as well as the
accretion and modification of planetary bodies in the solar system. The metamorphic
effects of shock on meteorite samples such as deformation, brecciation, local melting and
phase transformations, provide essential records of impact processes that can be
investigated in a large number and variety of samples. Understanding shock effects on
primary features in meteorites remains an important reason for further detailed studies of
shock metamorphic processes in meteorites.
Shock metamorphic features and shock-induced melt veins have been the study of
many investigations since (Fredriksson et al. 1963) interpreted black veins as shock-
induced melt. The veins form by localized melting and commonly contain large
polycrystalline clasts of high-pressure minerals such as ringwoodite and majorite
surrounded in a matrix of very fine-grained high-pressure minerals that crystallized from
the melt at high pressure (Chen et al. 1996). In our initial work of melt veins in
Sixiangkou (L6, $6) (Chen et al. 1996; Chen et al. 1996; Sharp et al. 1996), we
discovered a melt-vein assemblage consisting of magnesiowtistite plus majorite. This
assemblage, which is the stable liquidus assemblage observed in experiments from
approximately 23 to 25 GPa and 2000 °C (Agee et al. 1995), allowed us to estimate
crystallization pressures and temperatures for the first time. This provided the first
evidence that melt-vein assemblages could be similar to those of a static high-pressure
experiments (Sharp et al. 1995; Chen et al. 1996; Chen eta]. 1996; Chen eta]. 1996;
Sharp et al. 1996; Sharp et al. 1996). The applicability of phase equilibrium data to the
interpretation of shock-induced melt veins can only be tested by a detailed study of melt-
vein mineralogy to see how high-pressure assemblages vary as a function of shock
conditions inferred from other indicators. We have used transmission electron
microscopy (TEM), analytical electron microscopy (AEM), scanning electron
microscopy (SEM), electron microprobe analysis (EMA) and optical petrography to
characterize the mineralogy, microstructures, and compositions of melt veins and
associated high-pressure minerals in shocked chondrites and SNC meteorites. In the
processes, we have gained a better understanding of what melt veining can tell us about
shock conditions and we have discovered new mineral phases in chondritic and SNC
meteorites
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CosmochemistryNAG5-7285FinalReport ThomasG.Sharp
2. Proposed Research Objectives
The primary objective of this research was to use microstructural and
microanalytical techniques to characterize shock-induced melt veins and associated high-
pressure minerals that occur in chondritic meteorites ranging from $3 to $6 shock stages.
The data was intended to provide a better understanding of the formation and
crystallization of melt veins and the transformation mechanisms and kinetics of
associated forming high-pressure minerals.
A) Look at mineralogy and microstructures of melt-veins in a suite of L4 through L6
chondrites ranging from shock grades $3 through $6 to determine how crystallization
and assemblages are controlled by intensity of shock metamorphism.
B) Investigate the chemical, mineralogical, and microstructural heterogeneities within
melt veins to elucidate the extent of melt-mixing and the range of crystallization
processes in melt veins and pockets.
C) Use static high-pressure experiments to produce examples of equilibrium and
quench crystallization for comparisons with the melt veins of naturally shocked
samples using transmission electron microscopy (TEM) and scanning electron
microscopy (SEM).
D) Characterize the mineralogies, compositions, and defect microstructures of
polycrystalline aggregates of ringwoodite, wadsleyite, and majorite associated with
melt veins in $6 samples. The microstructural and microanalytical data will be used
to understand the mechanisms and kinetics of phase transformations that occur during
shock metamorphism of chondrites.
E) We will investigate the extent of back transformations of metastable high-pressure
minerals to better understand the thermal histories of samples after shock release.
This information will be interpreted in terms of post-shock cooling histories and the
relation between post-shock heating and peak-shock pressures.
Our actual research results deviate somewhat from those proposed because of some very
important research that we conducted on new SiO2 minerals in Shergotty. Our research
results for the grant period are summarized below and published scientific papers are
attached.
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3. Results
Melt-vein mineralogy and microstructure vs. shock stage
Although shock-induced deformation and transformation features have been used to
classify chondrites by shock stage (S 1-$6) and provide an estimate of shock pressures
(St6ffler et al. 1991), the conditions required to form high-pressure minerals are not well
understood. Shock-induced melt veins, common in L5 and L6 chondrites, contain high-
pressure mineral assemblages that can constrain crystallization conditions that occurred
during shock (St6ffler et al. 1991; Chen et al. 1996; Sharp et al. 1997). Melt vein
assemblages may provide an alternative shock-pressure calibration, depending on when
melt veins crystallize during shock events. We have examined 11 melt-vein-bearing L
chondrites that range from shock stage $3 to $6 to determine how melt-vein texture and
mineralogy correlate with the shock stage. These samples include: Roy, La Landa, Gifu,
Umbarger, Waconda, Pinto Mountains, Beaver, Kunashak, Nakhon Pathon, Tenham, and
Ramsdorf. Shock effects in olivine and plagioclase were characterized petrographically to
determine the shock stage. Black veins were characterized by optical microscopy, Field-
emission scanning electron microscopy (FESEM), Raman spectroscopy and transmission
electron microscopy (TEM). Qualitative microanalyses of the melt-vein minerals were
obtained using energy dispersive X-ray spectroscopy (EDS) combined with both FESEM
and TEM.
Nine of the samples were previously classified as $3 though $6 (St6ffler et al. 1991).
We find that the shock stages for most of these samples agree with the previous grade
determination if we ignore the transformation features that occur in or adjacent to melt
veins. However, considering transformation effects associated with melt veins, we find
that three of the samples contain evidence for higher shock stages. Roy, previously
classified as $3 (St6ffler et al. 1991), contains well-developed maskelynite around melt
veins suggesting $5. Ramsdorf, previously classified as $4 (St6ffler et al. 1991), and $6
(Yamaguchi et al. 1996) shows strong mosaicism in olivine and the presence of
maskelynite near melt veins suggesting $5. Umbarger, previously classified as $4
(St6ffler et. al 1991), contains well-developed maskelynite and shock-melted plagioclase
near melt veins as well as deep-blue ringwoodite in melt pockets. Based on St6ffler's
classification, Umbarger is $6. The remaining samples are $4 (La Landa, Gifu, Waconda,
Kunashak, and Nakhon Pathon,), $5 (Beaver, Pinto Mountains and Ramsdorf) and $6
(Tenham).
Textural variations
Low-grade veins: Roy ($3), Gifu ($4), and Nakhon Pathon ($4) have melt-veins that
are dominated by metal-sulfide surrounding angular silicate fragments. These small
silicate grains (< 500 nm to > 50 lam) suggest that melt-vein formation involved
CosmochemistryNAG5-7285FinalReport ThomasG.Sharp
cataclasisand frictional melting of metal sulfide. Roy containsevidencefor multipleveiningevents,with oneveinshowingevidencefor bothsilicateandmetal-sulfidemelts.
Intermediate-grade veins: Kunashak ($4), La Landa ($4) and Ramsdorf ($5) all have
very similar melt veins that are predominantly silicate with irregularly shaped blebs and
rounded droplets of metal-sulfide. The irregular shapes suggest of the metal-sulfide liquid
in the veins was in contact with solid-silicate grains in a partially molten silicate matrix.
Rounded metal-sulfide droplets indicate immiscible metal-sulfide and silicate liquids.
High-grade veins: Umbarger ($6) and Tenham ($6) contain melt veins and pockets
with predominantly round metal-sulfide droplets in a silicate matrix, indicating
immiscible metal-sulfide and silicate liquids. However, variations in the proportions of
silicate to metal-sulfide liquids suggest multiple veining events. The habits of the silicate
gains vary from vein rims to cores in both samples. Cores have granular textures whereas
the vein margins have acicular textures suggestive of a quenched margin.
High-pressure minerals in melt veins
Roy: The small amount of silicate melt in Roy ($3) crystallized to for an assemblage
of ringwoodite plus majorite. Based on the phase diagram of Allende (Agee et al.
1995)this represents near liquidus crystallization between about 18 and 23 GPa. These
conditions are similar to those found in $6 samples.
Ramsdorf" The silicate melt in Ramsdorf ($4) contains glass and clinoenstatite. The
clinoenstatite shows no evidence of twinning and is probably the high-pressure form,
which is stable up to about 16 GPa in enstatite-forsterite system. This assemblage
indicates rapid quench and crystallization between about 10 and 16 GPa.
Umbarger: Umbarger contains both high-pressure lithologies. The melt veins and
pockets contain many (10 to 50 lam) grains of polycrystalline ringwoodite that represent
transformation from olivine at pressure in excess of 16 GPa and possibly much higher.
Apparent maskelynite that occurs adjacent to melt veins is actually polycrystalline
material with the hollandite structure. The melt-vein matrix contains akimotoite
((Mg,Fe)SiO3-ilmenite), ringwoodite and augite where augite is clearly the last mineral to
crystallize. Because akimotoite is not a liquidus phase in chondritic or peridotitic
compositions, its occurrence suggests large supercooling. The sequence of crystallization
suggests a pressure decrease from 18-25 GPa to < 18 GPa with supercooling.
Tenham: Tenham also contains both high-pressure lithologies. The polycrystaline
lithogy includes ringwoodite, majorite, akimotoite and (Mg,Fe)SiO3-perovskite (Tomioka
and Fujino 1997). These polycrystalline phases represent phase transitions at pressures in
excess of 22 GPa and possibly much higher. The melt-vein matrix contains assemblages
that include akimotoite plus ringwoodite at melt vein margins, majorite plus
magnesiowtistite (and magnetite) as the predominant assemblage and majorite plus
ringwoodite in some core regions. These assemblages correspond to crystallization of
most of the veins at 23 - 25 GPa, but with super cooling of the margins between 18 and
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25 GPa. The majorite-plus-ringwoodite assemblage records a lower pressure (18-23 GPa)
in some melt vein cores.
The textures of melt veins vary systematically as a function of shock grade. The melt
veins in Roy reached temperatures > 950 °C, sufficient to melt the Fe-Ni-metal-sulfide,
but only a small amount of silicate was melted. The cataclastic textures of sulfide-rich
veins indicates mechanical mixing of silicate components and frictional heating to
produce mixed silicate and sulfide melt veins. The intermediate and high-grade veins
reached significantly higher temperatures, resulting in more silicate melting. Variations in
mineral habits across melt veins suggest variable thermal histories for the different parts
of the melt veins.
Although the melt-vein mineralogy is useful in constraining the pressure temperature
conditions of crystallization, there is considerable debate about the relationship between
crystallization pressure and the equilibrium shock pressure. If a melt vein cools through
the liquidus during the period of equilibrium shock pressure, a melt-vein or pocket should
contain one assemblage corresponding to that pressure. If melt'veins in various samples
crystallized at the equilibrium shock pressure, melt-vein assemblages should vary
systematically with shock stage. However, if crystallization of melt veins occurs during
adiabatic decompression, a range of crystallization pressures would be recorded in a
single sample and there should be no systematic variation of melt-vain mineralogy with
shock grade. There does not seem to be a systematic variation in the crystallization
pressures with shock stage in the samples studied here. For example, Roy ($3) contains
majorite plus ringwoodite (18-23 GPa) that also occurs in Tenham whereas Ramsdorf
($4) contains a lower-pressure assemblage (10-16 GPa). Both Umbarger and Tenham
contain evidence for varying crystallization pressures within single melt veins. The melt-
vein assemblage in Umbarger is consistent with disequilibrium crystallization during
rapid cooling and decompression with crystallization starting as high has 25 GPa and
ending below 18 GPa. The melt-vein assemblages in Tenham are dominated by the
liquidus assemblage of majorite plus magnesiowiastite (23-25 GPa), whereas the margins
are represent supercooling at similar pressures (18-25 GPa). The majorite-plus-
ringwoodite assemblage represents near liquidus crystallization at lower pressure (18-23
GPa), indicating that at least some of the crystallization was accompanied by
decompression.
Pressure Constraints from the Survival of Metastable Phases
Because melt-vein crystallization pressures may be lower than equilibrium shock
pressure, we need additional methods of constraining the equilibrium shock pressure. The
post-shock temperature of a given sample, which is related to equilibrium shock
pressures, can be constrained by the survival of metastable minerals. Phases such as
MgSiO3-perovskite, MgSiO3-ilmenite are unable to survive high temperatures at low
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pressure so they are sensitive to post-shock temperatures. At one atmosphere, MgSiO3-
perovskite and akimotoite (MgSiO3-ilmenite) vitrify at approximately 750 K and 973 K,
respectively. The Tenham L6 chondrite, which contains akimotoite and MgSiO3-
perovskite, is unlikely to have experienced post-shock temperatures above 750 K. The
Acfer 040 L5-6 chondrite, which contains (Mg,Fe)SiO3-ilmenite and vitrified
(Mg,Fe)SiO3-perovskite, experienced a post-shock temperature less than 973 K, but
greater than 750 K. Based on the calibration of equilibrium shock pressure and estimates
of post-shock temperature (St6ffler et al. 1991), Tenham experienced an equilibrium
shock pressure less than about 40 GPa, which is consistent with the maximum
crystallization pressures of 25 GPa, based on melt-vein assemblages. Acfer 040 appears
to have experienced an equilibrium shock pressure less than 48 GPa. In this case,
akimotoite and (Mg,Fe)SiO3-perovskite crystallized from the melt-vein liquid at a
pressure above about 18 GPa to well over 25 GPa. The Perovskite-akimotoite-
ringwoodite assemblage indicates crystallization during rapid cooling and
decompression. Because the metastable minerals occur in the hottest part off the sample
(the melt-vein), the pressure constraint provided by the survival of metastable minerals
may be significantly higher than the actual equilibrium shock pressure. Pressure
constraints from survival of metastable phases and from crystallization suggest
equilibrium shock pressures that are lower than those based on the pressure calibration of
(St6ffler et al. 1991).
Hollandite-structured maskelynite
Maskelynite is an optically amorphous form of shocked plagioclase that is generally
considered to be a diaplectic glass. In highly shocked samples there is commonly
evidence of flow (St6ffler et al. 1991; El Goresy et al. 1997). which suggests that some
plagioclase vitrifies through melting rather than through pressure-induced amorphization.
Such melted material has been used to argue that "maskelynite" is not diaplectic glass (El
Goresy et al. 1997). Crystalline "maskelynite" with the hollandite structure has been
discovered in the L6 chondrites Sixiangkou (Gillet et al. 2000) and in Tenham (Tomioka
et al. 1999). In one case, the material was characterized by Raman and X-ray diffraction
(Gillet et al. 2000) while in the other (Tomioka et al. 1999), TEM was used. We used
TEM to characterize the microstructures of the hollandite-structured plagioclase in
Sixiangkou and Tenham in order to constrain the origins of this new high-pressure
mineral.
Electron imaging and diffraction of the maskelynite near a melt vein in Sixiangkou
show that the maskelynite that occurs more than about 100 lum from the melt vein is
completely amorphous on a nanometer scale. However, maskelynites adjacent to and
within melt veins in both Sixiangkou and Tenham are full of crystals, ranging from 20 to
70 nm, that occur in an amorphous matrix (Fig. 1). These crystals are regular in shape
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Cosmochemistry NAG5-7285 Final Report Thomas G. Sharp
and randomly oriented. These crystals are beam sensitive and readily vitrify during TEM
imaging at high magnification. This semi-crystalline maskelynite occurs both in large
grains and within melt veins (Fig. 1). The vein filling morphology indicated that some of
this material was liquid at the time of vein formation.
e -_ ; z "
100 nm
Figure 2. HRTEM image of nanocrystalline hollandite (left) in an amorphous matrix from Tenham. Bright
field image (right0 showing a vein of hollandite structured plagioclase in Tenham.
Selected-area electron diffraction patterns consist of spotty rings made up of many
Bragg reflections (Fig. 2). The quality of the ring patterns indicates that the crystallites
are randomly oriented and very small. The patterns consist of up to 12 rings, all of which
can be indexed, within error (- 5%), as the hollandite structure. The observed d-spacings
match calculated d-spacings for hollandite, using the unit cell parameters of (Gillet et al.
2000). For Sixiangkou, 11 out of 12 rings are definitively indexed, with the remaining
ring being within error of two of the calculated d-spacings (510 and 411). For Tenham, 8
out of 10 rings are definitively indexed with the remaining two rings being within error of
two d-spacings each (240, 310) and (321,510). Although indexing of one or two closely
spaced lines is problematic, the fit of the remaining diffraction rings confirms that this
rstalline material has the hollandite structure.
Figure 2 SAED pattern of hollandite structured plagioclase in Sixiankou.
CosmochemistryNAG5-7285FinalReport ThomasG.Sharp
Qualitative EDS measurements on the TEM from Sixiangkou are consistent with this
material being an albite-rich plagioclase with a minor orthoclase component. This is in
agreement with electron microprobe analyses of the maskelynite in Sixiangkou
(Ab80An 12Or8) and in Tenham (Ab75An 15Or 10) •
The fact that the hollandite-structured maskelynite only occurs very near melt veins,
indicates that the transformation of plagioclase to the hollandite structure requires high
temperatures. Based on the majorite-magnesiowtistite assemblage in the melt veins of
Sixiangkou (Chen et al. 1996) the melt veins crystallized at approximately 2000 °C. The
very similar matrix assemblage in Tenham indicates similar crystallization conditions. As
in other high-pressure transformations in shocked chondrites, high temperatures are
required for the reaction kinetics to be fast enough for transformation on the short time
scales of shock.
The origin of the mixed glassy and nanocrystalline hollandite may be the result of
either partial crystallization of a high-density liquid or glass or the result of partial
amorphization of the polycrystalline hollandite-structured plagioclase. The random
orientations and regular rounded shapes of the hollandite crystallites are not consistent
with partial amorphization of hollandite after pressure release. In such post-shock
vitrification the high-pressure phase would likely be cut by veins of amorphous material,
as in the post-shock amorphization of high-pressure SiO2 polymorphs (Sharp et al. 1999).
Such vitrification leaves crystalline remnants that commonly have identical or nearly
identical orientations. The randomly oriented hollandite grains in an amorphous matrix
suggests that the hollandite crystallized from either a glass or a liquid at high pressure.
Veins of plagioclase composition material with nanocrystalline hollandite (Fig. 2)
indicate that at least some the hollandite crystallized from a liquid during shock.
However, complete melting of plagioclase within and adjacent to melt veins, at the same
time that the melt veins were liquid, is inconsistent with the sharp boundaries commonly
observed between "maskelynite and melt vein matrix. The melting of plagioclase in and
adjacent to melt veins may occur after the initial melting and partial solidification of the
melt vein.
The presence of plagioclase with the hollandite structure is consistent with the lower
pressure of the crystallization pressures of Sixiangkou and Tenahm (23 -25 GPa and
2000°C). Experimental studies of NaAISi308 at high pressure (Liu 1978) indicate that
the calcium ferrite structure plus stishovite, rather than hollandite, are stable above 23
GPa.
Post-stishovite Si02 polymorphs in Shergotty
Shock metamorphism is characterized by deformation microstructures and high-
pressure minerals and glasses. Quartz is a important indicator of shock metamorphism
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because it forms planar deformation features (PDFs) (Chao 1967; Gratz et al. 1988) and
because it transforms into the high-density polymorphs such as coesite and stishovite
(Chao et al. 1960; Chao et al. 1962; Chao 1967; St6ffler 1972; St6ffler and Langenhorst
1994). Experimental studies (German et al. 1973; Sekine et al. 1987; Tsuchida and Yagi
1989; Tsuchida and Yagi 1990; Kingma et al. 1995; Dubrovinsky et al. 1997) and
theoretical studies (Tse et al. 1992; Belonoshko et al. 1996; Karki et al. 1997; Teter et al.
1998) of SiOz at high pressure have shown that there are structures more dense than
stishovite, known as "post-stishovite" phases. Natural examples of dense SiO2 phases
were discovered in Shergotty and interpreted as "post-stishovite" structures that were
produced during the shock metamorphism of Shergotty (El Goresy et al. 1998; E1 Goresy
et al. 1998; Sharp et al. 1998). The dense SiO2 was initially thought to have the c_-PbO2
structure (El Goresy et al. 1998), but subsequent diffraction experiments have shown
more complex structures (El Goresy et al. 1998; El Goresy et al. 1998; Sharp et al. 1998;
Sharp et al. 1999). Here we combine field-emission scanning electron microscopy
(FESEM), powder X-ray diffraction (XRD), transmission electron microscopy (TEM),
selected area electron diffraction (SAED) to characterize two new SiO2 structures and
discuss these polymorphs in terms of the shock pressure experienced by Shergotty.
Silica in Shergotty mostly occur as large (> 150 lam) wedge-shaped grains typical of [3-
tridymite. They are embedded in clinopyroxene or between clinopyroxene, mesostacis,
and "maskelynite". Each grain is surrounded by radiating cracks that initiate at the
surfaces of the silica grains and penetrate deep (up to 600 tam) in the Shergotty matrix
(Fig. 3a). The radiating cracks are similar to those reported from ultra-high pressure
metamorphic rocks around coesite grains and are indicative of a large volume increase
after decompression. The individual silica grains consist of mosaics of many domains
(10-60 tam), many displaying an orthogonal intergrowth of two or more sets of lamellae
with different brightness in back-scattered electron images (Fig. 3b). Electron microprobe
analyses show that the lamellae and lamellae-free areas are almost pure SiO2 with minor
amounts of Na20 (0.40 wt. %) and A1203 (1.14 wt. %). A 120 tam disc containing a large
silica (> 60 tam) grain was cored out with a high-precision diamond micro-drill for
successive X-ray and TEM investigation.
Our initial electron diffraction data fit a post-stishovite structure similar to o_-PbO2.
Since the initial investigation we have obtained diffraction patterns from three distinct
zone axes, providing seven diffraction vectors to constrain the structure of this SiO,
polymorph (Table 1). The corresponding d-spacings cannot be indexed using any
structures of low-pressure SiO2 polymorphs, including tridymite. Similarly, the
diffraction data are inconsistent with coesite, stishovite and the known post-stishovite
structures: CaCl2-type, baddelyite -type, modified baddelyite (SBAD) and ct-PbO2
structures. However, the data nearly fit a synthetic orthorhombic Pbcn structure (German
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Cosmochemistry NAG5-7285 Final Report Thomas G. Sharp
et al. 1973) and a calculated orthorhombic structure (Pca2_) except for the 1.97 ]_ and
3.41 ,_ reflections.
Figure 3. BSE images of SiO2 showing radiating cracks in surrounding maskelynite and cpx, (a). FESEM
BSE images (b), show a fine lamellae microstructure in the SiO2 grains.
Since our diffraction patterns are consistent with orthorhombic symmetry, we used the
Pbcn and Pca21 structures as starting points to refine new cell parameters and d-spacings
(Table 1.). Our data fit the refined Pbcn and Pca21 unit cells in terms of d-spacings,
pattern symmetry, interplanar angles and angles between zone axes. However, the
systematic absences expected for the Pbcn structure provide the best match to the
extinctions in our data. We conclude that the SiOz phase in Shergotty, initially described
as c_-PbO2, (El Goresy et al. 1998), is a dense orthorhombic structure that fits the Pbcn
space group and has cell parameters a = 4.17/_, b = 5.12,4, and c = 4.55/_, and density
9 = 4.18 g/cm 3.
d-obs Pbcn Pca21 ref-Pbcn ref-Pca21
hkl d-calc hkl d-calc hid d-calc hkl d-calc
4.54* 001 * 4.50 100" 4.49 001 * 4.55 100" 4.55
3.41" 011"3.25 110 3.25 011"3.40 i10 3.40
3.22 110 3.17 011"3.17 110 3.23 011"3.23
3.09" 101"3.11 101"3.11 101"3.07 101"3.07
2.62 111 2.59 111 2.59 111 2.63 111 2.63
2.28 002 2.25 200 2.25 002 2.28 200 2.28
1.97 121 1.88 121 1.88 121 1.97 121 1.97
Table 1. Measured d-space data compared to calculated d-spacings for Pbcn, Pca2t and refined Pbcn,
Pca2_ unit ceils. Extinct reflections which occur in diffraction patterns by double diffraction, are indicated
by stars.
Powder X-ray diffraction data were collected from a single SiO2 grain using a
diffractometer with a rotating anode generator, capillary collimating system and CCD
area detector. The beam was collimated to 0.1 mm diameter and focussed onto a single
60-1am SiO2 grain. The diffracted X-rays were collected on a 512 x 512-pixel area
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CosmochemistryNAG5-7285FinalReport ThomasG.Sharp
detectorsetat threefixed 20 settings(15°, 25° and 30°). To make the relative intensities
of the reflections more representative, the sample was rotated during data collection.
The silica grain contains amorphous material which produces a broad halo at 20 = 8 °-
12 °. A total of 18 reflections were collected from the silica grain (Table 2). Some of these
(2.974(6), 2.023(4), 1.950(8), and 1.568(5) _) belong to stishovite, but most of
reflections could not be assigned to any known silica polymorph. A number of the
observed reflections are close to reflections obtained from quenched ot-PbO2-1ike phase
synthesized by (German et al. 1973), but several reflections (for example, 4.309(4),
2.767(3), 2.459) could not be indexed as the _-PbO/-like structure. Instead we indexed all
observed reflections (except a small broad reflection at 2.639(6)) in terms of a monoclinic
lattice with the cell parameters a = 4.375(1), b = 4.584(1), c = 4.708(1), 13= 99.97(3), p =
4.30(2) g/cm 3. The density of this phase appears to be slightly higher than the density of
stishovite (p = 4.28 g/cm 3, PDF #451374). The lattice parameters for the new phase are
closely related to those of the baddeleyite - type structures. Moreover, 16 observed
reflections of the new natural silica phase could be explained by the baddeleyite-type
structure. Exceptions are the weak unindexed reflection at 2.639(6) and the reflection at
2.974(6), (which is forbidden for the baddeleyite structure). The latter reflection
corresponds to the (110) reflection of stishovite (100% intensity reflection of stishovite).
While quantitative analysis of the intensities of reflections of the new silica phase is
difficult due to the strong preferred orientation and diffuse halo, the calculated intensities
for SiO2 with baddeleyite structure match the observed ones qualitatively very well
(Table 2).
TEM and SAED investigations of the grain used for X-ray diffraction indicate the
presence of the orthorhombic structure (Pbcn) described above, stishovite and an
unidentified material that probably corresponds to the baddelyite-type structure. The
three silica polymorphs are intergrown, forming a polymineralic grain. Some areas have a
distinct lamellar microstructure with orthogonal crystalline lamellae (20-100 nm wide)
cut by amorphous veins. Other regions are mostly amorphous with minor amounts of
crystalline material. Amorphous veins occur throughout the SiO2 and especially in the
orthorhombic phase.
The presence of multiple SiO2 structures is consistent with the complexity of post-
stishovite SiO2 phases (Belonoshko et al. 1996; Dubrovinsky et al. 1997; Karki et al.
1997; Teter et al. 1998). At P > 47 GPa, stishovite transforms to the CaCI2 structure
which transforms to a higher density-phase (_-PbO:, modified _-PbO2, or SBAD
structures) at 70 to 80 GPa. A polyphase mixture of stishovite and other high-density
SiOz structures in Shergotty is consistent with the fact that there are numerous post-
stishovite structures that have very similar energies (Teter et al. 1998). One of which, the
CaC12 structure, transforms to stishovite during quench. Similarly, the SBAD transforms
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5. References
Agee, C. B., J. Li, et al. (1995). "Pressure-temperature phase diagram for the Allende
meteorite." Journal of Geophysical Research 100(B9): 17725-17740.
Belonoshko, A. B., L. S. Dubrovinsky, et al. (1996). "A new high-pressure silica phase
obtained by molecular dynamics." American Mineralogist 81((5-6)): 785-788.
Chao, E. C. T. (1967). "Shock effects in certain rock-forming minerals." Science 156:
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